1. Field of the Invention
This invention relates generally to the field of semiconductor device manufacturing and, more particularly, to a method and apparatus for integrating multiple sample plans.
2. Description of the Related Art
Advanced process control (APC) systems are often used to coordinate operation of processing tools used to fabricate a semiconductor device. The processing tools may include photolithography steppers, etch tools, deposition tools, polishing tools, rapid thermal processing tools, ion implantation tools, and the like. Wafers (or wafer lots) are processed in the tools in a predetermined order and each processing tool modifies the wafers according to a particular operating recipe. For example, a photolithography stepper may be used to form a patterned layer of photoresist above the wafer. Features in the patterned layer of photoresist correspond to a plurality of features, e.g. gate electrode structures, which will ultimately be formed above the surface of the wafer.
The APC system includes a variety of devices for collecting data indicative of the physical state of one or more wafers before, during, and/or after processing by the processing tools. The collected data indicative of physical state of the wafer is typically referred to as wafer state data. The collected wafer state data may then be provided to the APC system, which may use the collected wafer state data to characterize the wafer and/or to detect faults associated with the processing. For example, a mean critical dimension associated with the various features may be indicative of a performance level of devices formed on the wafer and/or the wafer lot. If the wafer state data indicates that the mean critical dimension associated with the feature, e.g., a gate electrode feature, is on the lower end of an allowable range for such feature sizes, then this may indicate that the device formed on the wafer may exhibit relatively high performance levels. Higher performance devices may be sold at a higher price, thereby increasing the profitability of the manufacturing operation. However, the wafer state data may indicate that devices formed on the wafer and/or wafer lot have a relatively low performance level or are faulty if the mean critical dimension is near an upper end of the allowable range or falls outside of the allowable range.
Wafer state data may be collected by sensors incorporated within a processing tool, such as scatterometers, ellipsometers, and the like, in which case the wafer state data is referred to as in situ wafer state data. The in situ wafer state data may include measurements of a temperature of the wafer, a thickness of a layer of material formed above the wafer, a critical dimension of a feature formed above the wafer, or other characteristic parameters. Wafer sampling by the in situ sensors can increase the time spent by the wafer in the processing tool and so, in order to maintain a desired throughput, sensors usually perform gross metrology in which a small and/or isolated region on each wafer is sampled with relatively low accuracy.
Wafer state data may also be collected by devices external to the processing tool, in which case the wafer state data is referred to as ex situ wafer state data. The ex situ wafer state data may include a thickness of a layer formed above the wafer, a critical dimension (CD) of a feature formed above the wafer, and the like. For example, an integrated metrology tool, i.e. a metrology tool that is coupled to a processing tool, may be used to collect ex situ wafer state data from a subset of the wafers that have been processed in the processing tool. Relative to sensors included within the processing tool, integrated metrology tools typically perform higher accuracy measurements and/or measurements at a higher granularity. However, at least in part to maintain a desired throughput, the integrated metrology tools perform these measurements on a smaller number of wafers and/or on a smaller area on the wafer. For another example, a stand-alone metrology tool, i.e. a metrology tool that is physically separate from the processing tools, may be used to collect ex situ wafer state data from a subset of the wafers that have been processed in the processing tool. Compared to integrated metrology tools, stand-alone metrology tools typically perform higher accuracy and/or higher granularity measurements, but on a smaller number of wafers and/or on a smaller area of the wafer.
Conventional APC systems treat the wafer state data collected by different metrology tools as independent data sets, which may limit the ability of the conventional APC system to characterize wafers. Treating the wafer state data sets independently may also reduce the efficiency of a fabrication facility controlled by a conventional APC system. For example, a sensor, an integrated metrology tool, and a stand-alone metrology tool associated with a processing tool may each measure a mean critical dimension associated with various features formed above a wafer lot by performing measurements on the same, overlapping, and/or different sites on each wafer. Thus, the mean critical dimension may be determined using redundant information from the sensor, the integrated metrology tool, and/or the stand-alone metrology tool. Moreover, since the integrated and stand-alone metrology tools typically take longer to perform measurements, the redundant information may be acquired at a substantial cost in time and/or throughput.
The present invention is directed to addressing the effects of one or more of the problems set forth above.